1. Field of the Invention
The present invention relates to a control apparatus for a vehicle having a continuously variable transmission mechanism and a clutch mechanism in a power transmission path.
2. Description of the Related Art
In a continuously variable transmission (CVT) equipped in a power transmission path of a vehicle, there are a belt-type continuously variable transmission, a toroidal-type continuously variable transmission and so on. A continuously variable transmission mechanism incorporated in the belt-type continuously variable transmission has a primary pulley attached to an input shaft, a secondary pulley attached to an output shaft, and a drive belt wound around these pulleys. It controls a speed ratio continuously by changing a diameter of a loop of the drive belt. A continuously variable transmission mechanism incorporated in the toroidal-type continuously variable transmission has input discs attached to an input shaft, output discs attached to an output shaft and power rollers sandwiched between input discs and output discs. It controls a speed ratio continuously by changing a contact radius of the power rollers against each disc.
A torque capacity of the belt-type continuously variable transmission is determined based on a friction force between the pulleys and the drive belt, which is set depending on a magnitude of a hydraulic pressure supplied to hydraulic chambers of the pulleys. A torque capacity of the toroidal-type continuously variable transmission is determined based on a shearing force of a traction oil between the discs and the power rollers, which is set depending on a magnitude of a thrust power for thrusting the discs in a axial direction. When a torque inputted to the continuously variable transmission mechanism is larger than the friction force or the shearing force, a slip of the drive belt or the power rollers occurs.
In order to prevent the slip of the drive belt, etc., and to avoid damage of the continuously variable transmission mechanism, it is needed to increase the friction force of the drive belt or the shearing force of the traction oil. However the increase of the friction force, etc., causes an increase in internal resistance of the continuously variable transmission mechanism, which becomes a factor of a decline of power transmission efficiency. Moreover, raising the operating hydraulic pressure to increase the friction force etc causes an increase of power loss occurring in an oil pump, which might lower fuel economy. In an effort to solve these problems, it is desirable to set the torque capacity of the continuously variable transmission mechanism low within the range that a slip of the driving belt and the power rollers does not occur.
Japanese Unexamined Patent Application Publication No. 2003-227562 (Prior Art 1) and Japanese Unexamined Patent Application Publication No. 2004-245290 (Prior Art 2) describe a continuously variable transmission having a clutch mechanism which a torque capacity is smaller than that of a continuously variable transmission mechanism and making the clutch mechanism slip earlier than the continuously variable transmission mechanism when an excess torque develops in the power transmission path. There is described reducing the torque capacity of the continuously variable transmission mechanism until a slip occurs while protecting the continuously variable transmission mechanism by providing the clutch mechanism and detecting a slip of the clutch mechanism. There is further described a device to achieve a high-precision clutch control of the clutch mechanism by monitoring the relationship between a clutch pressure supplied to the clutch mechanism and a clutch capacity of the clutch mechanism obtained thereby.
The continuously variable transmissions described in the Prior Art 1 and the Prior Art 2 calculate and learn a clutch pressure of the clutch mechanism based on an input torque to the continuously variable transmission mechanism and based on the supposition that the continuously variable transmission is operating steady state or a state free from a large torque deviation. If a torque capacity of the clutch mechanism is set based on the input torque to the continuously variable transmission mechanism when the torque capacity of the continuously variable transmission mechanism is controlled based on a predetermined lower limit hydraulic pressure such as a vehicle braking condition, it is difficult to operate the clutch mechanism as a fuse clutch effectively, i.e. the clutch to disconnect the transmission of the torque by slippage in the case where the excess input torque is applied. On vehicle braking, a difference of the torque capacity of the continuously variable transmission mechanism and that of the clutch mechanism might become large, because the torque capacity of the continuously variable transmission mechanism is increased in order to adapt the braking torque. In such a situation, there might be an unnecessary slip of the clutch mechanism, even if the torque capacity of the continuously variable transmission mechanism has a margin that attempts to accommodate the braking torque. Performing a learn control of the clutch mechanism which the vehicle is a braking state causes a decline in a learning accuracy and a decline of the control accuracy of the clutch mechanism, because it is difficult to estimate the braking torque inputted from the drive wheels.
An embodiment of the present invention is described of avoiding an unnecessary slip of the clutch mechanism for protecting a continuously variable transmission mechanism and avoiding a decline of the learning accuracy of the clutch mechanism.